Speedo Issue

barthel · Feb 11, 2011

Hey Guys,

I've done a bunch of searches for speedo issues and can't quite find the answer I'm looking for, so I thought I might ask the experts.

My speedo is off by 9 or 10 mph at highway speeds. At 2500rpm's I'm indicating 85mph but only going 75 or 76.

I know that it's possible to change the gear on the trans, but am not sure which one to use.

It's on a '94 GT convertible with an AODE automatic trans.

Any help would be much appreciated.

Pops Fun · Feb 12, 2011

Hi
Have the rear end gears been changed or anything else that might affect the speedo??
Here is a site that offers speedo gears to change the output.
Ford Speedometer Gears for Automatic and Standard Transmission

A more expensive approach is a Ford speedo cal which is electronic for around $130.00 I just did mine.

barthel · Feb 12, 2011

Pops,

I don't know if the gears have been changed or not. I'm fairly certain that the tires are smaller (245/45HR/17) and that could be part of what's causing it.

I've only had this car about 6 months and am still learning all the nuances of the Mustang. This car may have been lowered (not sure) but I haven't jacked up the ass end to check the gear ratios yet. I would assume it's stock only because not much mechanically looks like it's been done to this car.

Chythar · Feb 12, 2011

The VSS (vehicle speed sensor) is the part that tells the computer and the speedometer your speed. If your speedometer was bouncing up and down or not working, you might have other issues. But if it appears to be working correctly other than being more off the faster you go, the gear on the VSS is not correct for the gears in the rear end. It is likely you have a gear ratio of 3.73 or 4.10 in the rear end, since those cannot be completely corrected by changing the plastic gear on the VSS. If this is the case, you'll need a SpeedCal or the more expensive Ford version in order to properly correct your speedometer.

Jack up the rear end and

cjrpony · Feb 12, 2011

barthel said:
Pops,

I don't know if the gears have been changed or not. I'm fairly certain that the tires are smaller (245/45HR/17) and that could be part of what's causing it.

I've only had this car about 6 months and am still learning all the nuances of the Mustang. This car may have been lowered (not sure) but I haven't jacked up the ass end to check the gear ratios yet. I would assume it's stock only because not much mechanically looks like it's been done to this car.

245/45/17 is the optional tire size and is correct for your car. So check the rear gears.

toyman · Feb 12, 2011

Chythar said:
The VSS (vehicle speed sensor) is the part that tells the computer and the speedometer your speed. If your speedometer was bouncing up and down or not working, you might have other issues. But if it appears to be working correctly other than being more off the faster you go, the gear on the VSS is not correct for the gears in the rear end. It is likely you have a gear ratio of 3.73 or 4.10 in the rear end, since those cannot be completely corrected by changing the plastic gear on the VSS. If this is the case, you'll need a SpeedCal or the more expensive Ford version in order to properly correct your speedometer.

Jack up the rear end and

If the car came from the factory with 3:27 gears (Axle code E it will have a 7 tooth drive gear AODE). If the gears are now 3:73 a 21 tooth driven gear is absolutely correct. No ifs, ands or buts.

Chythar · Feb 13, 2011

toyman said:
If the car came from the factory with 3:27 gears (Axle code E it will have a 7 tooth drive gear AODE). If the gears are now 3:73 a 21 tooth driven gear is absolutely correct. No ifs, ands or buts.

I thought that all AODE's came with an 8-tooth drive gear. Learn something new every day. You are correct, with a 7-tooth drive gear the 21 tooth is exactly right.

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